In addition, many communities are trying to minimize future rate increases by deploying both traditional gray and newer “green” stormwater control measures. In general, gray infrastructure collects and diverts excess runoff, while green infrastructure keeps it from entering the collection system. Gray measures like pipes and basins aren't typically integrated into the surrounding infrastructure, while green measures often become part of the street, building, or green space in which they lie. Consequently, green infrastructure delivers benefits beyond controlling runoff that are difficult to quantify for comparison to conventional alternatives.
Another budgeting challenge is that low-impact designs like green roofs, pervious pavements, and rain gardens lack the decades of engineering and construction experience associated with curbs, inlets, and pipes. Nor has EPA promulgated standards for such designs, which is why you may have received a request from the agency in August for performance data on your operation's low-impact designs.
Two years ago, the Indianapolis Department of Public Works retained our firm (Williams Creek Consulting Inc.) to create a Green Infrastructure Master Plan to manage runoff in the combined sewer watershed portion of the city. Since then, we've identified, designed, and/or implemented pilot projects to study the cost and effectiveness of green measures.
Figure 1 (top) shows the anticipated reduction in rainfall volumes for varying amounts of green measures in varying soil types. A 6% to 8% BMP:ISA ratio manages 20% to 40% of impervious surfaces with minimal disruption to existing or planned infrastructure.
To prioritize project implementation, we reviewed aerial photographs to identify open spaces adjacent to impervious surfaces and the department's GIS to avoid existing utilities. Control measures applied at the 6% to 8% BMP:ISA ratio to those projects remove 80% of the annual runoff volume — 1 to 2 billion gallons annually — from the collection system, saving $1 million to $2 million annually at the wastewater treatment plant based on current rates.
Figure 2 (bottom) was developed for the climate conditions modeled in Figure 1 and soils with an infiltration rate of 0.13 inches/hour. Results indicate that cost efficiencies are greatest where control measures are phased into application, with each phase intentionally undersized in case later phases aren't needed to achieve the desired management goal.DEVELOPING YOUR COST/SQUARE FOOT OF IMPERVIOUS SURFACE MANAGED
There are various databases (see Web extra for links) to help estimate installation costs for individual control measures, but they don't tie costs to the measures' common denominator: impervious surface. Follow these three steps to develop metrics that apply to your operation's unique situation.
Determine management goal. Are you going to control annual runoff volume infiltration or single events? The goal defines how much runoff a measure is expected to infiltrate or otherwise remove from the collection system.
Choosing annual infiltration requires defining a typical year rainfall data set and modeling using a continuous simulation; results are expressed as percent volume removal or gallons per year.
Events are expressed as a “water quality event.” They range between 0.75 and 1.25 inches of runoff, but vary based on local and state climate and watershed conditions.